In a quick post to her site Lorna Mitchell reminds us that you can modify the options on the internal PHP streams (like for HTTP requests), more specifically to add a "Bearer" token value for OAuth2 integration.

For OAuth2, all I had to be able to do was to send an Authorization header with my web request from PHP. My second-favourite way of making API calls from PHP is to use PHP's stream handling, so I did that. It's not code you see very often but it's super-simple and it works on every PHP platform I've tried so far

She gives a few lines of code to show how to modify the options to add in the token value, basically using an array of options and a call to stream_context_create to create the context. This can then be used in future calls (like the file_get_contents she shows) to automagically send this information along with the request.